Cardiovascular system question

[equilibr8]

Why is blood pressure low in a dehydrated person? I know that blood volume decreases and blood pressure also decreases, but if blood volume decreases, shouldn't pressure increase according to PV=nRT?

Does blood pressure decrease because there is less force per area P = F/A along the blood vessels? If so, how do you know to use the equation P = F/A and not PV=nRT?

Thanks.

---

Members don't see this ad.

 

[nebraskadoc]

first, I think PV = nRT is only used for gases, not fluids

second, when you reduce V in PV = nRT, you are reducing the volume the gas takes up (such as the size of the container). in the human body, you are not reducing the "container" (the body), just the amount inside of it.

so if you DO think of it in terms of PV = nRT (which im still not sure about)... maybe think it in terms of n, as n (amount of fluid) goes down, you are reducing the pressure.

EDIT: http://en.wikipedia.org/wiki/Blood_pressure - about half way down may have some formulas worth looking at if you really want to understand it

 

[thestrokes14]

Why is blood pressure low in a dehydrated person? I know that blood volume decreases and blood pressure also decreases, but if blood volume decreases, shouldn't pressure increase according to PV=nRT?

Does blood pressure decrease because there is less force per area P = F/A along the blood vessels? If so, how do you know to use the equation P = F/A and not PV=nRT?

Thanks.

No. PV=nRT only applies to a dilute gas (or an ideal gas). It has nothing to do with liquids (and blood is even a non-ideal liquid).

Basically, blood volume is correlated with pressure. Just memorize that.

 

[enTropeeeeeeeee]

PV = nRT applies to ideal gases.

 

[crafty14]

Its probably best not to think of it in terms of pv=nrt since that is for ideal gasses only and will end up confusing you. However, if you like to think of it that way, volume will remain constant (because this represents the volume of your circulatory system, not the fluid). thus, as n (moles of fluid) goes down, pressure also goes down to compensate.

But as the other poster said, better to just know volume and pressure are directly proportional.

 

[equilibr8]

Thanks, everyone! This is great!

 

[SN2ed]

Thread moved. These types of questions belong in the Study Q&A forum.

 

[whiteshadodw]

No. PV=nRT only applies to a dilute gas (or an ideal gas). It has nothing to do with liquids (and blood is even a non-ideal liquid).

Basically, blood volume is correlated with pressure. Just memorize that.

THIS. blood volume increases meaning pressure has to increase in order to get all the blood pumped throughout the body. all the blood in the body has to circulate through the heart and lungs in a certain amount of time.

does it take longer to empty a 5 gal water bottle or a 1 gal water bottle? now, does it take longer to empty and refill a 5 gal water bottle or a 1 gal water bottle? ok, now consider this, you have to empty and fill both water bottles are the same time. how are you going to achieve this? you can't just let gravity do its work, not gonna work like that. so you have to increase pressure to pump more blood through the circulatory system.